The present invention is a curable organosiloxane-polyisobutylene composition comprising 20 to 87 weight percent of a polyisobutylene comprising at least two alkenyl groups per molecule, 0.4 to 70 weight percent of a polyorganosiloxane comprising at least 2 silicon-bonded alkenyl groups per molecule, 7 to 50 weight percent of a treated reinforcing silica filler, an organohydrogensiloxane crosslinker in an amount sufficient to cure the composition of, and a platinum-group metal hydrosilation catalyst in an amount sufficient to effect curing of the composition.
Platinum-group metal curable polydiorganosiloxane compositions which cure to form elastomers are well known in the art and the consistency of such compositions can vary from that of a flowable liquid to a high-consistency gum. These curable polydiorganosiloxane compositions have found many applications in areas requiring good material flexibility and strength within a wide temperature range, high stability to heat and chemical degradation, and areas requiring good electrical insulating ability. Such areas of applications include automotive hoses, protective boots, and spark plug wiring and boots. In addition such curable polysiloxanes, particularly those commonly referred to as liquid silicone rubber (LSR), are useful as protective coatings for fabrics and as optically clear compositions suitable for forming face mask. Although such curable polydiorganosiloxane compositions when cured typically present a significant barrier to chemical and moisture permeation, in some applications it is desirable to further reduce the chemical and moisture permeation of the cured composition.
The present inventors have found that the incorporation of a polyisobutylene comprising at least 2 alkenyl groups per molecule into a platinum-group metal curable polyorganosiloxane composition results in cured elastomers having improved resistance to gases and water vapor permeation. The alkenyl groups of the polyisobutylene allows the polyisobutylene to be crosslinked by a hydrosilation-type reaction during curing of the composition thereby providing for a cured polydiorganosiloxane-polyisobutylene elastomer having good physical properties such as tear and elongation.
Northrup et. al., U.S. Pat. No. 3,328,482, teach a silicone release agent which can consist essentially of a vinyl end-terminated polydiorganosiloxane, a polymeric material consisting of butylene having a molecular weight over 400, organohydrosiloxane, and chloroplatinic acid.
French Publication No. 2,393,831, describes curable compositions comprising hydroxy end-terminated polydiorganosiloxanes and polybutylenes with a number-average molecular weight ranging from 300 to 2500. The presence of the polybutylene in the composition is reported to reduce permeability of the cured compositions to gases and vapors.
Alexander et al., EPO Patent Publication No. 0-220-809A2, describe a flowable elastomer forming composition comprising a polydiorganosiloxane having silicon-bonded hydroxyl groups and polybutylene. The composition is reported to have a lowered water vapor permeability.
Katsuno, JP 63-199277, describes a sealant composition comprising an polyorganosiloxane, inorganic filler, and polyisobutylene. The composition is reported to have enhanced resistance to temperature changes and improved water proofing.